A plug or plenum fan is the term used to describe the application of backward inclined or airfoil fans housed in large plenums. The fan typically consists of a single-inlet impeller assembly with an inlet bell-mouth. The orifice is mounted flush to one side of the plenum, such that the orifice and shaft of the fan are generally in the direction of the flow. Both draw-through and blow-through applications are used. For draw-through applications, the fan and plenum are located downstream of the heating and cooling coils. For the blow-through applications, the fan and coil sections are reversed. In the draw through case, the fan pressurizes the plenum and one or more discharge ducts are attached at any of the side-walls. For most packaged units however, the discharge is attached directly downstream of the fan/plenum section. This section may include passive mufflers, filter sections and additional coils (blow-through). For cases where passive mufflers are supplied, an additional settling section is required which adds to the overall length of the system. An inlet section is attached at the fan/plenum interface; this may also include the same components as those described for the discharge section.
To control the noise from air handling units, duct active noise control (ANC) systems are starting to be employed in air distribution systems. An ANC system basically requires the sensing of the noise associated with the fan for distributing the air, producing a noise canceling signal and determining the results of the canceling signal so as to provide a correction signal to the controller producing the noise canceling signal. There is a time delay associated with sensing the noise and producing a canceling signal. This time delay necessary for the canceling to take place equates to the minimum flow path distance in the system required between the reference, or input, noise sensor and the loudspeaker. Additional space is required between the loudspeaker and the error sensor which adds to the flow path distance in the system. The space limitations in existing buildings severely limits the retrofitting or replacement of existing equipment with equipment using conventional ANC approaches due to the system length requirements. The employing of an active noise control device would eliminate the need for both the downstream settling and passive muffler sections. However, conventional active noise control configurations would also add considerable length to the system, on the order of six to eight feet.
The fan is asymmetrically located within the plenum with an offset such that the centerline of the fan is biased towards one of the corners of the plenum. This offset places the fan close to two walls of the plenum such that they effectively act like the scroll of a centrifugal fan, diffusing the flow and providing a more efficient operation. By asymmetrically locating the fan, as described, a corner opposite one in which the fan is located can be the location of the outlet with a partition defining a part of the discharge path as well as a part of the effective scroll for the fan. The partition can serve as a location of at least a portion of the active noise control structure thereby minimizing the system length increase due to the active noise control structure.
It is an object of the invention to attenuate noise at the inlet or discharge of a plug fan using active noise control.
It is another object of this invention to provide optimized performance in combination with a small package size.
It is a further object of this invention to locate the discharge duct relative to the fan so as to increase aerodynamic efficiency. These objects, and others as will become apparent hereinafter, are accomplished by the present invention.
Basically, a plug or plenum fan is located near two walls of the plenum such that they tend to act as the scroll for the fan. Additionally, a partition separates the fan from an opposite corner further enhancing the formation of a scroll while providing a flow passage and a location for at least a part of the active noise control structure.